The present invention relates to a radio frequency amplifier and to a television tuner including such an amplifier.
It is a requirement, for example in cable and terrestrial television tuners, to provide a front end having a high intermodulation distortion performance (such as IIP3 and IIP2) and a low noise figure. FIG. 1 of the accompanying drawings illustrates a known example of a radio frequency amplifier formed in an integrated circuit and comprising a long tail pair of first and second transistors 1 and 2 whose collectors are connected to a Gilbert cell 3 forming a mixer or a current steering stage to provide automatic gain control (AGC). The bases of the first and second transistors 1 and 2 are connected to first and second differential or balanced inputs IN+ and INxe2x88x92 and via resistors 4 and 5 to a bias voltage Vbias 2. The emitters of the transistors 1 and 2 are connected via emitter resistors 6 and 7, respectively, to a tail current source, which comprises a transistor 8 whose base is connected to a bias voltage Vbias 1 and whose emitter is connected via a resistor 9 to ground. The Gilbert cell 3 is provided with load resistors 10 and 11 connected to a supply line VCC.
When used as the first stage of, for example, a television tuner, the input signal is supplied to the first input IN+ via a coupling capacitor and the second input INxe2x88x92 is grounded via a capacitor 12. The long tail pair converts the input voltage to a differential current emerging from the collectors of the transistors 1 and 2, which current drives the Gilbert cell 3.
The emitter resistors 6 and 7 have the same resistance. In order to maximise the signal handling of the amplifier, the product of the resistance of each of the emitter resistors 6 and 7 and the direct current flowing through each of the resistors has to be maximised. This also reduces the third harmonic distortion in the signal current emerging from the collectors of the transistors 1 and 2. However, because the amplifier is driven in an unbalanced mode with the signal voltage being supplied to the first input IN+ and the second input INxe2x88x92 being grounded at signal frequencies, the amplifier exhibits some residual second harmonic distortion, which is a function of the impedance of a source connected to the first input IN+ and a complex function of transistor parameters.
In order to avoid this problem, the amplifier inputs may be driven differentially as shown in FIG. 2 of the accompanying drawings. An external transformer 15 has a primary winding with one end connected to receive the input signal and the other end grounded and a secondary winding connected between the bases of the transistors 1 and 2. Although this improves the second harmonic distortion performance of the amplifier, the transformer 15 is a relatively large and costly item and has to be provided externally of the integrated circuit.
GB 1 372 240 discloses an arrangement for controlling the line oscillator frequency in a television receiver. Part of the circuit comprises Darlington pairs of transistors connected as a long tail pair.
GB 1 075 436 discloses a unity gain buffer amplifier intended for audio frequencies and based on Darlington transistors connected as a long tail pair.
WO 95/01049 discloses a television IF amplifying arrangement which includes a long tail pair of transistors whose bases are driven by the emitters of another long tail pair. Both long tail pairs have emitter degeneration resistors and a common current source.
U.S. Pat. No. 4,498,053 discloses a current amplifier for use in an audio noise reduction system. The circuit includes a long tail pair of transistors driven by the emitters of two further transistors.
According to a first aspect of the invention, there is provided a radio frequency amplifier comprising a long tail pair of first and second transistors, a third transistor whose base is connected to a first input of the amplifier and whose emitter is connected to the base of the first transistor, and a fourth transistor whose base is connected to a second input of the amplifier and whose emitter is connected to the base of the second transistor, the third and fourth transistors having first and second collector resistors, respectively.
The first and second transistors may have first and second emitter resistors, respectively.
The long tail pair may comprise a common current source for the emitters of the first and second transistors.
The third and fourth transistors may have third and fourth emitter resistors, respectively.
The amplifier may comprise first and second capacitors connected between the bases and collectors of the third and fourth transistors, respectively.
The amplifier may comprise first and second feedback resistors connected between the bases and collectors of the third and fourth transistors, respectively.
The second input of the amplifier may be connected to ground via a third capacitor.
The collectors of the first and second transistors may be connected to a Gilbert cell.
According to a second aspect of the invention, there is provided a television tuner comprising an amplifier according to the first aspect of the invention.
It is thus possible to provide an amplifier having good signal handling ability and intermodulation distortion performance while maintaining a low noise figure. Such an amplifier may be used as or in the front end of a television tuner, for example feeding a mixer or automatic gain control stage.